Earthmoving and construction work machines often employ hydraulic steering systems that control steering functions and operation. These steering systems often provide pressurized fluid to hydraulic actuators arranged to change a steering angle of front or rear wheels to steer the work machine.
On a conventional work machine, a steering system includes two steering actuators that control the steering angle of the wheels. During steering, the system typically extends one actuator while retracting the other. Extending and retracting the actuators often may be accomplished by introducing pressurized fluid into a head end of one actuator and a rod end of the other actuator. Thus, both actuators provide a steering force to change the wheel steering angle and steer the work machine.
One example of a known system that provides conventional steering control is disclosed in U.S. Pat. No. 5,520,262. The system includes a three position directional valve disposed between a pump and a pair of steering cylinders. The valve and fluid lines are arranged to provide fluid to both steering cylinders to implement either a right or left turn. Accordingly, when turning, fluid is fed to the head end of one steering cylinder and the rod end of the other steering cylinder.
While the system disclosed in the '262 patent may be effective for providing steering control, the system may also introduce some inefficiencies. These efficiencies may occur because, in addition to hydraulic steering control, a conventional work machine includes many additional systems and implements requiring hydraulic power. For example, work machines, such as wheel loaders, may include a loading bucket moveable with hydraulic powered actuators. While steering, conventional steering systems may draw fluid away from these additional hydraulically powered systems and implements, thereby reducing the fluid power available to those systems and implements.
In addition, conventional steering control systems may provide more steering force than is necessary, thereby resulting in additional inefficiencies. For example, one or more hydraulic pumps may generate the fluid pressure used for steering. These pumps may be driven by an engine, such as an internal combustion engine. Therefore, operating the pumps to provide more fluid pressure than may be required may increase the load on the engine, and likewise, increase fuel consumption.
Another known hydraulic system that may allow implement control while steering may be configured to provide fluid power to only one of two steering actuators, thereby maintaining fluid and fluid power for operation of the other implements. If the fluid power at the powered actuator is insufficient to properly steer the work machine, the steering system may direct fluid to both actuators. However, the known system is complex, includes many components, and may be difficult to implement. For example, the system does not use a main valve that controls flow to all the chambers. The many components necessary to operate the known steering system may increase production costs and may reduce reliability.
Yet another system that provides fluid power first, to only one of two steering actuators, and then if required, to a second steering actuator is disclosed in U.S. Pat. No. 5,193,637. The '637 patent discloses a system that uses a control valve that connects one chamber of a first steering piston cylinder to a fluid pump. A directional valve is used to direct fluid to a chamber of a second steering piston cylinder when fluid pressure exceeds a given amount. The system disclosed in the '637 patent requires multiple valves in communication with the steering piston cylinders and, therefore, may be overly complex and expensive.
What is needed is a steering system that may be less complex, less expensive, more efficient, and more compact than previous steering systems, yet still may provide desired pressure during steering to additional hydraulic systems and implements. The system disclosed and described herein may overcome one or more of the problems in the prior art.